This invention relates to apparatus for automatically testing electrical storage batteries and, in particular, relates to apparatus for automatically testing lead-acid storage batteries of the type conventionally used for lighting, starting and ignition in the automotive field.
The desirability of adequate methods and apparatus for quickly, economically and simply determining the acceptability of electrical storage batteries has long been known; the desire for such methods and apparatus for use with lead-acid storage batteries of the type generally used for lighting, starting and ignition in automotive applications has been particularly accute. Automotive batteries are required to operate under widely varying loads and at widely varying temperatures. However, the most severe condition which an automobile battery in typical use must meet is the starting of an automobile engine at low temperatures. The electric starter of an automobile is typically the largest single load placed on the battery and at low temperatures the load is increased even further because of the increased viscosity of the lubricants used in automobile motors. Additionally, automobile batteries are typically least able to deliver large amounts of current to heavy loads at low temperatures. It is, desirable, of course, for a motorist to be able to quickly and easily determine whether the battery of his automobile is in sufficiently good condition that it will be able to start that automobile under low temperature conditions without having to await the occurrence of those conditions. It is important that methods and apparatus for testing the acceptability of automotive batteries be reliable not only so that motorists may be given a true indication of when their batteries need to be replaced, but also because such methods and apparatus are used by manufacturers and sellers of automotive batteries to determine whether such batteries need to be replaced under applicable warranty provisions. It is apparent that those in the automotive battery business desire to have neither the dissatisfied customers and attendant bad will resultant from supplying batteries to customers that pass the test measures used but will not perform acceptably and, in particular, will not start their customers' automobiles under low temperature conditions, nor the unnecessary expense resultant from replacing under warranty provisions batteries that fail the test measures but would, in fact, perform acceptably in use.
In large measure because of the criticallity of the low temperature performance of automotive batteries, the Battery Council International and the Society of Automotive Engineers have developed a standard for automobile batteries known as the "cold cranking rating". The cold cranking rating of an automobile battery is measured in amperes and is the current which the battery is able to deliver for a period of thirty seconds at a temperature of zero degrees Farenheit without the battery terminal voltage falling below a predetermined level, which level in the case of conventional, six-cell, twelve-volt lead-acid storage batteries is 7.2 volts.
One method for testing the condition of lead-acid batteries which has been used for many years has been to subject the battery to a constant current load for a specific period of time and measure the voltage across the battery at the end of that period with the battery still subject to the load. If the battery voltage at the end of the period was below a predetermined level, a hydrometer then was used to determine the state of charge of each one of the battery cells by measuring the specific gravity of the acid within it. Typically with such tests, if the temperature of the battery was other than 80 degrees Farenheit, in order to achieve accurate results both the current load test and the specific gravity readings had to be corrected for changes in temperature. These measurements, the temperature correction, and the ultimate determination of whether or not a battery was an acceptable one, i.e., whether it needed to be replaced, required some amount of training and skill. Further, it required removal of the battery vent caps for access to the liquid within the battery cells.
However, many automotive batteries commercially available today are designed to operate for their entire lifetime with no or very infrequent replacement or replenishment of the liquid contained within them and, thus, are provided with either no means for access to the liquid contained within the battery or only limited means for such access suitable for only infrequent use. For batteries such as these, testing apparatus must be provided which does not require access to the interior of the battery. In general, the only way to test a battery is then to measure its electrical parameters at the battery terminals. A simple electrical load test with monitoring of the terminal voltage as described above might be used alone, but then each battery would have to be fully charged prior to commencing the test, an undesirable alternative because of the time required to properly fully charge a battery.
Other battery testing methods and apparatus have, of course, been proposed over the years as alternatives to the load test described. Some such apparatus are described in U.S. Pat. No. 3,873,911 to Keith S. Champlin dated Mar. 25, 1975, entitled "Electronic Battery Testing Device" and in a paper by Keith S. Champlin purportedly presented at the 1975 SAE Off-Highway Vehicle Meeting and available from the Society of Automotive Engineers, Inc., 400 Commonwealth Dr., Warrendale, Pa. 15096, and such devices are sold under the name Electronic Battery Tester from Motorola, Inc., Automotive Products Division, 9401 West Grand Ave., Franklin Park, Ill. 60131, Models 8TE 2003 and 7BT1181W. Each of these apparatus is electronic in nature and purports to measure the dynamic conductance of the battery which is said to be proportional to the maximum power available from the battery. The Motorola, Inc. Electronic Battery Tester includes provisions for indicating to the user a direct reading of the maximum power available from the battery or the battery terminal voltage and for comparing the measured maximum power with a value determined from the manufacturer's cold cranking rating to give the user a "pass-fail" indication. Apparatus are purportedly described in U.S. Pat. No. 3,753,094 to Haruhisa Furuishi, et al., dated Aug. 14, 1973, entitled "Ohmmeter for Measuring the Internal Resistance of a Battery and Directly Reading the Measured Resistance Value" for measuring the internal resistance of a battery. However, these apparatus and methods continue to have disadvantages associated therewith. For example, the Motorola, Inc. Electronic Battery Tester referred to above may give incorrect results immediately after the battery has been subject to a high level charging current or when a short exists in the battery.
In the development of methods and apparatus for testing batteries it is desirable to make effective use of as much information as may be obtained about the condition of the battery as possible but this, of course, must be done within certain constraints as it is undesirable that the testing apparatus be unduly expensive or that the testing procedure consume an unnecessarily long period of time. Further, in developing such methods and apparatus where a plurality of separate tests are performed on the battery it is highly desirable that the tests be structured in such a way as to prevent "looping", i.e., the repetition of one or more test steps during a test operation. Such repetition can be time consuming but can also result in a failure of the test apparatus to arrive at a definitive indication of battery acceptability if a condition should arise where the apparatus by normal operation was unable to get out of the loop and merely continuously repeated the tests in the loop. In the development of battery testing methods and apparatus it is also desirable to have the ultimate judgment of the acceptability of the battery subject to as little operator interpretation or judgment as possible. In particular, it is desirable that battery testing apparatus be capable of use by relatively unskilled personnel.